Generation of Series of Meter/Decimeter III type Bursts During Thermal Phase of Solar Flare by Meshalkina and Altyntsev – Community of European Solar Radio Astronomers – Win99Update – Stay Updated with the Latest Tech and Software News

Generation of Series of Meter/Decimeter III type Bursts During Thermal Phase of Solar Flare by Meshalkina and Altyntsev – Community of European Solar Radio Astronomers

It has been established that decimeter type III bursts are generated in the upper corona by electron beams. However, many questions remain about their sources to this day. Some were associated with the jets generated by regular solar flares, but most were found to be associated with weak energy release events. The purpose of this work is to determine the relationship of meter and decimeter III type bursts with the plasma eruption in the thermal phase of flare. We analyze in detail the June 29, 2012 flare, using multiwave observations. This event is unusual in that during a few minutes there is an outflow of thermal plasma with high temperatures up to 12 MK when there is still no response either in the microwaves or in hard X-rays.

Observations

The June 29, 2012 flare occurred at 04:09–04:16 UT (hereafter, Universal Time is used) in active region #11515; the flare class according to GOES is C4.6. Figure 1 presents light curves of the flare in X-rays, decimeter and microwave radiation.

EUV images show that energy release of the flare occurs in low loops (flare core in Figure 2) located at the footpoints of a large-scale loop and open field lines. A high loop is observed before the flare. At 04:09, compact brightenings appeared in the SDO/AIA images in the 304 Å channel. A minute later, at 04:10, plasma began to flow along open field lines directed from west to east in the plane of the sky (Figure 3). With time, brightness of these plasma jets increased. Another jet emerged in a high closed loop and gradually reached the remote western footpoint of the loop.

Figure 1. The June 29, 2012 event: a — GOES data, radiation fluxes (solid curve — 0.5–4 Å, dash-dot line — 1–8 Å); b — time profiles of HXR in the 25–50 keV RHESSI channels; c,d — dynamic spectra (HiRAS); e — integral intensity of radio emission from 50 to 550 MHz (solid curve). Velocities of heat wave fronts propagating along open magnetic field lines (see Figure 4) are indicated by asterisks. Dotted lines mark the time of the beginning of radio bursts, the dash-dot line on both panels indicates the beginning of a burst of hard X-rays (04:13:23).

Figure 2. Large-scale structure of a flare region. Background is the difference between the images captured at 04:12:11 and 04:09:35 in the AIA/SDO 131 Å channel. The flare core, the jet (spray), and the high loop, along which another jet moved, are shown.

The dynamics of the southern jet, along the trajectory of which the blue dashed line is drawn, is shown in the sequence of images in the 304 Å channel (see Figure 3). It can be seen from the brightness profiles (Figure 3) that during propagation of the hot plasma flow there is not a gradual increase characteristic of diffusion, but a steeping of ~ 6“ in size at the beginning of the jet, which can be interpreted as a quasi-stationary thermal front. The front profiles are seen the brightness distributions along the open magnetic lines (Figure 3). The front velocities reaches 300-500 km/s.

Figure 3. Images in the AIA/SDO 304 Å channel for the given time points. White dash-dot lines mark the horizontal cross-section of the image. The brightness profile along the cross-section is highlighted in white at the bottom of each panel. The blue dashed line connects fronts (brightness jumps) of the propagating jet. Along the X and Y axes, the values are given in pixels.

Discussion and Conclusion

In the event under study, hot plasma with temperatures above 10 MK appears at the initial stage of the flare, and a burst of non-thermal electrons is observed a few minutes later. A feature of this event is the heating of plasma to an abnormally high temperature in the flare core, as well as near footpoints of large-scale loops, which creates favorable conditions for studying heat and plasma propagation into the upper corona.

The SDO/AIA images suggest that part of the heated plasma moves from the hot core along open field lines, and another part propagates along high loops (closed in the field of view) toward remote footpoints. The behavior of EUV-brightness distributions was examined along the latitude-aligned open field line (see Fig. 2). For ~5 min from the appearance of the jet to the end of the impulsive phase, there were four sequentially propagating brightenings with a front width ~4–5 thousand km during plasma outflow (Fig. 3). The fronts separated from the hot core of the flare and moved at velocities 300–500 km/s, which at a plasma temperature 10 MK are 30–40 times lower than the thermal velocity of electrons, but several times higher than the thermal velocity of ions.

Slowdown of spreading of hot electron bunches in cold plasma was previously observed in laboratory experiments. In flares, this effect was first noted in [Batchelor et al., 1985; Rust et al., 1985]. A theoretical explanation for the formation of a heat jump between hot and cold plasmas moving at a velocity much lower than the thermal velocity of electrons was discussed by [Bardakov 1985]. Electrons scatter due to compensation of an outgoing charge by the counter flow of cold electrons since ions do not have time to move.

The June 29, 2012 flare saw no Neupert effect, i.e. this flare was a preheating event observed in soft X-rays before the impulsive phase, which was detected later in hard X-rays. When hot electrons of sufficiently high density fly apart along the magnetic field into an ambient plasma, quasi-periodically moving jumps in the density of hot electrons are formed, which can be associated with brightness fronts in the EUV images. The most energetic electrons overcome the electric potential jumps and generate the beams and, as results, pulses of decimeter emission.

The formation of such fronts is not observed when spreading the jet along a high loop. This confirms the continuity of the flux of hot electrons from the flare core. Perhaps the relative flux density of hot electron along the high loop is too small to induce a counter flux of electrons of the sufficiently high velocity.

Based on the recent paper: Altyntsev, A. T., Meshalkina, N. S. Heating Manifestation at the Onset of the 29 June 2012 Flare: 2024, Solar-Terrestrial Physics, 10 (3), P.11-17, DOI: 10.12737/stp-103202402.

References